1. Field of the Invention
The present invention relates to an image heating apparatus used with a copying machine, a printer and the like, and more particularly, it relates to an image heating apparatus suitable for color copying machines, color printers and the like.
2. Related Background Art
In fixing devices used with an electrophotographic image forming apparatus such as a copying machine, a laser beam printer and the like, a recording material to which a toner image was transferred is pinched between a pair of rollers so that the image is permanently fixed to the recording material with heat and pressure. Surfaces of the rollers used in such fixing devices are coated by material having good mold releasing ability, heat-resistance and anti-wear, such as fluororesin, silicone rubber or the like.
Particularly, in color copying machines having a purpose for providing good image quality, there is a tendency for using a fixing roller having a surface on which silicone rubber is coated. Further, in full-color copying machines, in order to obtain a multi-color image, different color toner images are formed on a recording material. In this case, during a fixing operation, since the toner images must be fully fused and mixed, unlike to mono-color copying machines in which each of rollers is constituted by coating a thin fluororesin layer on a metal core, rollers having substantial thickness and softness and called as "soft rollers" having large heat capacity must be used so that the toner can be heated and fused in a confined manner.
The toner used in this case must have good fusibility and mixing ability upon application of heat, and, thus, sharp-melt toner having low softening point and low fusing viscosity is usually used. However, when the above-mentioned features are used in the fixing device, there arises a problem regarding "overshoot temperatures" of the fixing roller and the pressure roller.
Now, factors for generation the overshoot and disadvantages caused by the overshoot will be explained. First of all, when such rollers having large layer thickness are used as the fixing roller and the pressure roller, the time for transmitting the heat from a heater disposed a center of the roller to the outer surface of the roller is increased. After the heater is turned ON by detecting the fact that the temperature of the roller surface is low by means of a temperature detecting element contacted with the roller surface, it takes a very long time to increase the temperature of the roller surface to a desired value by the transmission of the heat from the heater. In an image forming apparatus in which a process speed is slow and a sheet interval i.e. sheet-to-sheet distance (between adjacent recording materials) is long, as shown by the symbol "A" in FIG. 6, since the temperature of the roller surface is restored during the recording material sheet interval, it is not required that the temperature adjust temperature is set to a high value.
However, in a high speed image forming apparatus, as shown by the symbol "B" in FIG. 6, before the heat of the heater is transmitted to the roller surface, the temperature of the roller surface continues to be decreased. Thus, as shown, it is necessary to set the temperature adjust temperature (temperature at the initiation of the fixing operation) to a high value, such as 190.degree. C. That is to say, if the temperature adjust temperature of "B" is set to 165.degree. C as is in the "A", the temperature of the roller surface reaches a fixing lower limit temperature soon, with the result that the fixing operation cannot be effected.
Next, in observing the change in temperature when the copies continue to be obtained, as shown in FIG. 7, the temperature of the roller surface continues to be decreased until the temperature of the heater is transmitted to the roller surface. At that point (shown by (i)), the temperature of the roller surface is gradually restored due to the heat of the heater. In this case, since the temperature adjust temperature is 190.degree. C, as shown, the heater is tuned ON repeatedly until the temperature of the roller surface reaches 190.degree. C. Thus, if a large number of copies are obtained continuously, as shown, the temperature of the roller surface may be restored to the start temperature (temperature adjust temperature).
However, if the copying operation is stopped in the course of the gradual temperature increase (for example, at a point (ii)), a further recording material does not pass through a nip between the fixing roller and the pressure roller. As a result, since the heat on the roller surface is not absorbed by the recording material and the radiation of heat is not enhanced due to the stoppage of the rollers, the temperature of the roller surface is abruptly increased as shown by the symbol "a.sub.2 " (or "a") in FIG. 7. This is the overshoot. The amount of the overshoot is varied with the heat accumulating ability (heat accumulating amount) of the metal core and cover rubber layer of each roller and of any element(s) contacted with the roller(s) because the heater is repeatedly turned ON till the point (ii). In general, in the case where the rubber layer is made of the same material, the thicker the rubber layer, the greater the heat accumulating amount. Accordingly, in the above-mentioned color copying machines and color printers, since the rubber layer has great thickness, the amount of the overshoot becomes great.
The symbol "A" shown in FIG. 7 indicates the change in temperature of the roller surface during the first continuous copying operation after the copying machine is powered, and the symbol "B" shown in FIG. 7 indicates the change in temperature of the roller surface during the continuous copying operation started from a highest temperature point of the first overshoot. Regarding the "B", although the time duration between ON and OFF of the heater is shorter than that in the "A", the heat accumulating amount and the fixing lower limit temperature become greater than those in the "A", with the result that the amount of the overshoot becomes greater than that in the "A". If the continuous copying operation started from the highest temperature point of the second overshoot, the amount of the overshoot will be further increased.
Since the heat resisting ability of the silicon rubber used with the fixing roller is about 200.degree. C. to 230.degree. C. in the continuous operation, if such great overshoot occurs, the silicon rubber layer will be damaged. Further, the most undesired deterioration of image quality will occur. That is to say, not only if the temperature of the roller surface is too low, the cold offset will occur to cause the deterioration of image quality due to toner peel, but also if the temperature of the roller surface is too high, the sharp melt toner becomes hard to be separated from the heated rollers to cause the toner peel (called as "hot offset"), thereby deteriorating the image quality. And, the offset toner is gradually accumulated on the roller(s), thereby decreasing the service life of the roller(s).